ATM Chip Database

ATM xBR Service Segmentation & Reasembly Controller

The Bt8233 consists of five separate coprocessors (Incoming & Outgoing DMA, Segmentation,
Reassembly and xBR Traffic Manager), each of which maintains state information in shared, off-chip
memory. This memory is controlled by the SRC through the local bus interface, which arbitrates access to
the bus between the various coprocessors. These coprocessors, though they run off the same system clock,
operate asynchronously from each other. Communication between the coprocessors takes place through
on-chip FIFOs or through queues in local memory. Designed as a total solution for UNI 3.1 and TM4.0, the Bt8233 provides automated
segmentation and
reassembly processes in silicon. The Bt8233 service-specific features allow system
designers to accelerate specific protocol interworking
functions for applications like Frame Relay or LAN Emulation. In addition, the device's
unique
architecture enables advanced network-level functionality and topologies.

Advanced Architecture

The Bt8233's architecture is designed to minimize and control host traffic congestion. The host manages
the Bt8233 terminal using write-only control and status queues. The control queues are also isolated from
their associated data buffers via buffer descriptors, allowing the data buffers to hold payload data only. For
example, the host submits data for transmit by writing buffer descriptor pointers to one of 32 Transmit
Queues. These entries may be thought of as task lists for the ServiceSAR to perform. In addition, the
Bt8233 can perform ATM server functions for up to 32 clients. The Bt8233 enables control of traffic
congestion through mechanisms like Receive Buffer memory limitations (called firewalls) and through
explicit notification of congestion by the host. This architecture lessens the control burden on the host
system while minimizing PCI bus utilization by eliminating reads across the PCI bus from host control
activities. It also provides control points to manage congestion, which is critical for ABR.